In analytical chemistry, liquid and gas chromatography techniques have become important tools in the identification of chemical sample components. The basic principle underlying all chromatographic techniques is the separation of a sample chemical mixture into individual components by transporting the mixture in a moving fluid through a porous retentive media. The moving fluid is referred to as the mobile phase and the retentive media has been referred to as the stationary phase. One of the differences between liquid and gas chromatography is that the mobile phase is either a liquid or a gas, respectively.
In a gas chromatograph, typically, a supply of inert carrier gas (mobile phase) is continually passed as a stream through a heated column containing porous sorptive media (stationary phase). GC columns have also been known to comprise a hollow capillary tube having an inner diameter in the range of few hundred microns. A sample of the subject mixture is injected into the mobile phase stream and passed through the column. As the subject mixture passes through the column it separates into its various components. Separation is due primarily to differences in the volatility characteristics of each sample component with respect to the temperature in the column. A detector, positioned at the outlet end of the column, detects each of the separated components as they exit the column.
The analytical choice between liquid and gas chromatography techniques is largely dependent on the molecular weight of the compound being analyzed. Liquid chromatographs are capable of analyzing much heavier compounds than gas chromatographs. However, since gas chromatography detection techniques are more sensitive, they are preferred.
The advent of Supercritical Fluid Chromatography (SFC) provided a potential bridge between gas and liquid chromatography advantages, i.e., high sensitivity and heavier molecular weight samples. In SFC, a fluid heated above the critical point, is used as the mobile phase. Such fluid is passed under pressure through a media which differentially retains sample components. As the pressure of the mobile phase is increased, for example, from about 40 ATM to approximately 400 ATM, the sample being analyzed separates into its various components dependent upon the relative differential solubility of each component with the mobile phase. Since the mobile phase is a gas, detectors used in GC can be utilized, significantly enhancing detection sensitivity and selectivity.
SFC has been found to be primarily useful in the analysis of moderate molecular weight homologous series (M.W. 100 to 10,000) and some thermally labile molecules such as pesticides and pharmaceuticals. The problem with SFC, however, is the long period of time involved in conducting a sample analysis. Although GC techniques are faster than SFC techniques, extended periods of time can occur between GC analyses, which is also undesirable. Time between GC analyses is generally related to cooling of the inlet or oven used in a first test to a suitable temperature prior to starting a second test.
It has been known in the past to program temperature in gas chromatographic analyzation since separation of the sample components is due primarily to differences in the volatility characteristics of each component with respect to the temperature in the column. By raising the column temperature either in a constant linear fashion or in a variable non-linear fashion over a sufficient range of temperature one can assure high resolution detection of all sample components in a minimized time period. High resolution is assured because each component is emerging from the column at its optimum temperature. Since the highest temperatures occur at the end of a test, it is necessary to cool the chromatographic apparatus before beginning the next analysis.
As used herein the term resolution refers to the distinctness of graphed peaks generated by known detection apparatus, wherein each peak is representative of the detection of a sample component.
It has also been known in the past that the time required between temperature programmed GC analyses can be reduced by providing a coolant in various sections of the chromatographic equipment, such as the oven or the injection apparatus to bring the temperature in that section down to a desired level prior to beginning the next analysis. In addition it has been known during a GC analysis to utilize a temperature profile which has a portion below room temperature, particularly in the analysis of highly volatile components. Along these lines, various devices are known for cooling the oven or cooling the injection apparatus.
U.S. Pat. No. 4,269,608 -- Sisti et al. discloses an injector for providing a sample gas in a so-called "on column" injection technique. In on column injection, the sample gas is injected directly into the inlet end of the column and is not first passed through a splitter type device. In Sisti the injector apparatus is shown, in one embodiment, to have a coil which is said to be capable of drawing heat from the injector by passing a fluid at a suitable temperature therethrough.
The problem with these prior devices and techniques is that cooling and heating of the oven and injection apparatus is achieved independently of one another. During cool down between analyses, cooling of the oven can become a bottleneck because a greater amount of heat needs to be removed. Additionally, redundant hardware is required and non-optimal use of the cooling fluid can occur. Consequently, a need still exists for a GC apparatus which optimize coolant use, minimizes required hardware and which minimizes the time between analyses necessary for cool down.